Various optical scanners are known for such applications as data storage, bar code reading, image scanning (surface definition, surface characterization, robotic vision), and lidar (light detection and ranging). Referring to FIG. 1, a prior art scanner 50 generates a moving spot of light 60 on a planar target surface 10 by focusing a collimated beam of light 20 through a focusing lens 40. If the assembly is for reading information, reflected light from the constant intensity spot 60 is gathered by focusing lens 40 and returned toward a detector (not shown). To write information, the light-source is modulated. To cause the light spot 60 to move relative to the surface 10, either the surface 10 is moved or the scanner 50 is moved. Alternatively, the optical path could have an acousto-optical beam deflector, a rotating prism-shaped mirror, or a lens driven galvanometrically or by piezoelectric positioners. Scanners also fall into two functional groups, raster and vector. Both types generally use the same types of beam deflection techniques.
Higher-speed raster scanners use either spinning prism-shaped (polygonal cross-sectioned) mirrors or multifaceted spinning holograms (hologons). Performance parameters for these conventional beam deflection techniques are listed in Table 1. The discrete optics in these devices are generally attended by high costs for mass manufacture, assembly, and alignment.
(from The Photonics Design and Applications Handbook 1993, Laurin Publishing Co., Inc., p. H-449)
The performance parameters listed in Table 1 assume different levels of importance depending on the optical scanning application. For raster scanning to cover extended surface areas, the emphasis is on speed, area resolution, and scan efficiency. Wide bandwidth is needed if the surface is to be color-scanned. For applications requiring vector scanning of precise paths at high resolution, the optical system typically uses a monochromatic, focused spot of light that is scanned at high speed with low wavefront distortion and low cross-axis error. Optical data storage has been a prime application of this type of optical scanning.
In optical data storage media, information is stored as an array of approximately wavelength-size dots (cells) in which some optical property has been set at one of two or more values to represent digital information. Commercial read/write heads scan the media with a diffraction-limited spot, typically produced by focusing a collimated laser beam with a fast objective lens system as shown in FIG. 1. A fast objective lens, one with a high numerical aperture, achieves a small spot size by reducing Fraunhofer-type diffraction. The spot is scanned by moving an assembly of optical components (turning mirror, objective lens, position actuators) over the optical medium, either along a radius of a disc spinning under the spot or across the width of a tape moving past the head. The assembly moves in one dimension along the direction of the collimated laser beam. As the disk spins or the tape feeds, the line of bit cells must be followed by the spot with sufficient precision to avoid missing any bit cells. The fine tracking is achieved by servo mechanisms moving the objective lens relative to the head assembly. An auto-focus servo system is also necessary to maintain the diffraction limited spot size because the medium motion inevitably causes some change in the mean/medium separation with time. Proper focus adjustment is possible because the medium is flat and smooth. Such a surface reflects incident light in well-defined directions like a mirror. Light reflected from the medium is collected by focusing optics and sent back along the collimated beam path for detection.
Scanning by several spots simultaneously is used to achieve high data rates through parallelism in one known system called the CREO(copyright) optical tape system. One scanning device that avoids reliance on discrete optical elements to achieve scanning is described in U.S. Pat. No. 4,234,788. In this scanner, an optical fiber is supported rigidly at one end in a cantilevered fashion. The supported end of the fiber is optically coupled to a light emitting diode or photo diode for transmitting or receiving light signals, respectively. The fiber is free to bend when a force is exerted on it. The fiber can thus be made to scan when light from the light-emitting diode emanates from the tip of the fiber as the fiber is forced back and forth repeatedly. To make the fiber wiggle back and forth an alternating electric field, generally perpendicular to the axis of the fiber, is generated. The fiber is coated with a metallic film. A charge is stored on the film, especially near the tip, by forming a capacitance with a metallized plate oriented perpendicularly to the fiber axis (optically at least partly transparent). The stored charge makes the fiber responsive to the electric field.
A drawback of this device is the limit on the speeds with which the fiber can be made to oscillate. The device requires a series of elements to move the fiber: an external field-generating structure, a DC voltage source to place charge on the fiber coating, and an AC source to generate the external field. Another drawback of this prior art mechanism is the inherent problem of stress fractures in the fiber optics. Bending the fiber repeatedly places serious demands on the materials. Problems can arise due to changes in optical properties, changes in the mechanical properties causing unpredictable variation in the alignment of the plane followed by the bending fiber, the amplitude of vibration, the natural frequency of vibrations, and structural failure. Still another limitation is imposed by the need to place a conductor between the fiber tip and the optical medium to form the capacitance. This places another optical element between the fiber tip and the scanned surface and makes it impossible to sweep the tip very close to the scanned surface as may be desired for certain optical configurations.
Another prior art scanning device is described in U.S. Pat. No. 5,422,469. This patent specification describes a number of different devices to oscillate the end of an optical light guide or optical fiber. One embodiment employs a piezo-electric bimorph connected to the free end of a device to which the free end of an optical fiber and a focusing lens are attached. Reflected light is directed back through the fiber to a beam splitter which directs the reflected light out of the bidirectional (outgoing/return) path at some point along the fiber remote from the source of light. The above embodiment uses a simpler prime mover, a piezo-electric bimorph. However, the need for a focusing lens attached to the end of the fiber, by increasing the mass, imposes difficult practical requirements for high speed oscillation of the fiber. In addition, to achieve very small projected spot size requires a high numerical aperture at the output end of the focusing optics. It is difficult to achieve this with the conventional optics contemplated by the ""469 disclosure. Furthermore, the reciprocation of the fiber as described in the ""469 patent requires a multiple-element device. Friction between the motor and the fiber can cause changes in the optical properties of the fiber, and mechanical changes in the motor, the fiber, or the interface, that result in changes (which may be unpredictable) in the amplitude of oscillation or the resonant frequency of the motor-fiber combination (which might generate, or be susceptible to, undesired harmonics). Also, the process of assembly of such a combination of a motor and a fiber presents problems. Ideally, for high frequency operation, the device would be very small.
Common to all storage/retrieval devices is the need for greater and greater data rates. Increases in speed have been achieved by increasing the speed of scanning. However, there are practical limits, particularly with regard to the writing operation, relating to physical properties inherent in the optical media.
Also common to the applications of optical scanning technology is the need for great precision in the focus of the scanning light source and the return signal.
A controller for a scanning head images light emitted with a high numerical aperture from an output aperture using focusing optics with a one-to-one mapping of points in the output aperture plane onto points in the target plane. Light is conducted to the output aperture by a light guide. As a result of the one-to-one mapping ratio, light from the image is returned along the original path of the outgoing light and focused back onto the output aperture. The focus of the imaging optics are a controlled by maximizing the received light. The light guide has a directional coupler that taps received light and directs into a detector. The detector signal is applied to a control input through a low pass filter to obtain the focus control signal and applied to separate signal conditioning for data output. Alternatively, the controller could internally register itself for the possible states of received light and respond accordingly. That is, the data stream is generated and supplied to the controller along with the total amount of light. The controller, if it determines that there is an amount of light that corresponds to neither kind of data (e.g., a one or a zero), it adjusts accordingly. In this way, the same device is used for both reading the data and controlling focus.
According to an embodiment, the invention provides an optical scanning device. The device has a scanning head with a light output. A focusing element images light from the output onto a target surface. The focusing element is effective to image a first amount of return light returned from the target surface back to the output when the focusing element is properly focused and a second amount of the return light, returned from the target surface back to the output, when the focusing element is improperly focused. A detector in a return path of the light, inside the scanning head, generates an output responsive to the return light. A controller is programmed to control a configuration of the focusing element responsive to the output such that the focusing element remains properly focused by maintaining the return light at the first amount. In a variation, the scanning head includes an optical fiber or guide with the output being an end of the optical fiber or guide. In another variation, the light from the output diverges with a numerical aperture ratio of about 0.5. In still another variation, the focusing element has a 1:1 magnification. In still another variation, the scanning head includes an optical fiber and the output is an end of the optical fiber. In still another, the scanning head includes a directional coupler in series with the optical fiber or guide to divert light to the detector. In another variation, the scanning head includes multiple optical fibers or guides and the output is one of several outputs coinciding with respective ends of the optical fibers. In this embodiment, the scanning head includes directional couplers in series with the optical fibers to divert light to the detector or detectors.
According to another embodiment, the invention provides an optical scanner with a scanning head having an array of optical fibers or guides. Each fiber or guide extends to the outside of the scanning head. This results in an array of light output apertures coinciding with ends of the optical fibers or guides. Light from the output is imaged by a focusing element, with one-to-one mapping ratio, onto a target surface. The focusing element is such that the amount of return light from the target surface back to the output, when the focusing element is properly focused, is at a maximum and less when out of focus. A detector in a return path of light in the scanning head, generates an output responsive to the return light. A controller is programmed to control a configuration of the focusing element responsively to the output such that the return light is maintained at the maximum. In a variation, the light from the output diverges with a numerical aperture ratio of about 0.5. In another variation of this embodiment, the magnification of the focusing element is 1:1. In still another variation, the scanning head includes a directional coupler in series with at least one of the optical fibers or guides to divert light to the detector. The scanning head has a laser connected to direct light into the fiber or guide. According to another embodiment, the invention provides a method of controlling a focusing element of a scanning device. The method performs the following steps. Imaging, with a one-to-one mapping, an output of the scanning device onto a target surface to be read by the scanning device. Receiving light returned from an image of the output. Controlling a focus by determining a result of the step of receiving to maintain light received in the step of receiving at a maximum. According to a variation, the step of receiving includes receiving light at an input aperture which is identical to an output aperture from which the output in the step of imaging is emitted.
A light in single or multiple modes from a laser source is fed into one end of an optical fiber or waveguide designed so that this light diverges from the other end with a numerical aperture (NA) ratio determined by the fiber properties and tip configuration. A simple optical system with matching NA is used to focus the light emitted from the fiber tip to produce the desired scanning spot size. Light reflected from the scanned surface travels back along the fiber, from which it is channeled to a sensor for signal detection. The property of this system in which it re-images and collects the returned light through the fiber when the spot is focused on the scanned surface, means that an autofocus system for the scanner will use the total light returned from a given surface area as a feedback parameter. When the light returned is maximized, the system is in focus. The scanning may be achieved by moving the fiber and lens relative to the surface to be scanned. Other alternatives are moving the fiber tip relative to the optical axis and moving the lens relative to a fixed fiber tip.